1. Field of the Invention
The present invention relates to a battery state monitoring circuit which is capable of controlling the charging/discharging operation of a secondary battery, and a battery device using that circuit.
2. Description of the Related Art
A conventional battery device formed of a secondary battery is shown in a circuit block diagram of FIG. 2. For example, Japanese Patent Application Laid-open No. Hei 4-75430 discloses the structure of the power supply device shown in FIG. 2. In the structure, a secondary battery 201 is connected, to an external terminal xe2x88x92V0205 or +V0204 through a switch circuit 203 that limits the current, i.e. the switch circuit 203 acts as a current limiter. Also, a battery state monitoring circuit 202 is connected in parallel with the secondary battery 201. The battery state monitoring circuit 202 detects the voltage and current of the secondary battery 201. In any of: an over-charge state in which the secondary battery 201 has a voltage value higher than one voltage, an over-discharge state in which the secondary battery 201 has a voltage value lower than another voltage, and an over-current state in which the current flowing in the switch circuit 203 exceeds that of a given current value with the result that an external terminal xe2x88x92V0205 reaches a certain voltage, a charge/discharge inhibit signal is outputted from the battery state monitoring circuit 202 so that the switch circuit 203 turns off so as to suspend a charge current or a discharge current. Hereinafter, the states where the secondary battery 201 is in the over-charge state, in the over-discharge state or in the over-current state to stop the charging operation or the discharging operation are respectively called over-charge protective state, over-discharge protective state, or over-current protective state.
Further, another conventional example of a battery device having the battery state monitoring circuit is shown in a circuit block diagram of FIG. 3. Referring to FIG. 3, an over-charge detecting circuit 306, an over-discharge detecting circuit 307, an over-current detecting circuit 308, delay circuits 309, 310, 311, and a logic circuit 305 are combined together into a battery state monitoring circuit 202. In FIG. 3, a charger 301 is connected between external terminals +V0204 and xe2x88x92V0205, and when the voltage in the secondary battery 201 becomes equal to or more than an upper limit of a charging voltage, an over-charge detection signal is outputted to the delay circuit 309 from the over-charge detecting circuit 306, and when the over-charge detection signal continues for a given period of time or longer, the over-charge detection signal is outputted to the logic circuit 305 from the delay circuit 309. Also, a load 302 is connected between the external terminals +V0204 and xe2x88x92V0205. When, the secondary battery 201 becomes equal to or less than a lower limit of a discharging voltage, an over-discharge detection signal is output to the delay circuit 310 from the over-discharge detecting circuit 307, and when the over-discharge detection signal continues for a given period of time or longer, the over-discharge detection signal is output to the logic circuit 305 from the delay circuit 310. Also, when a discharge current that flows in the switch circuit 203 becomes equal to or more than an upper limit and the potential of the external terminal xe2x88x92V0205 becomes equal to or more than a given value, the over-current detection signal is outputted to the delay circuit 311 from the over-current detecting circuit 308, and when the over-current detection signal continues for a given period of time or longer, the over-current detection signal is outputted to the logic circuit 305 from the delay circuit 311. Upon inputting the over-charge detection signal, the logic circuit 305 outputs a charge inhibition signal to an FET-B 304, thereby being capable of suspending the charge current. Also, upon inputting the over-discharge detection signal or the over-current detection signal, the logic circuit 305 outputs the discharge inhibition signal to an FET-A 303, thereby being capable of suspending the discharge current.
In the conventional power supply device structured as shown in FIG. 3, it is possible to ensure a delay time until the charge current or the discharge current stops, that is, an over-charge detection delay time, an over-discharge detection delay time and an over-current detection delay time. Those delay times are required to prevent the malfunction caused by a temporal noise or the like.
However, the conventional power supply device structured as shown in FIG. 3 cannot ensure a delay time for returning the stopped charge current or discharge current, that is, an over-charge release delay time, an over-discharge release delay time and an over-current release delay time. For that reason, there arise the following problems.
For example, in the conventional battery device, a drawback arises when a pulse discharge occurs while charging, as shown in a timing chart of FIG. 9. First, a pulsed discharge current flows. When a voltage drop occurs in an internal impedance of the secondary battery 201 and the supply voltage becomes lower than the over-charge voltage temporarily, an over-charge protective state is instantaneously released. As a result, the charge current flows for the over-charge detection delay time. The above operation is repeated in accordance with the pulse. Therefore, a problem arises such that even if the voltage of the secondary battery 201 rises to the over-charge voltage or more, the battery state monitoring circuit cannot maintain the over-charger protective state at a given voltage, and the charging operation continues in a pulsed fashion.
Also, the conventional battery device suffers from a drawback immediately when the battery device is in the over-discharge protective state as shown in a timing chart shown in FIG. 11. First, the discharge current stops as soon as the over-discharge protective state is entered. Then, the battery voltage temporarily rises due to the parasitic coil component of the secondary battery 201 or the like and exceeds the over-discharge voltage. Then, the over-discharge protective state is released instantaneously. As a result, the discharge current is allowed to flow for the over-discharge detection delay time. The above operation is repeated in accordance with the over-discharge detection delay time, and oscillation occurs. Therefore, even if the voltage of the secondary battery 201 drops, the battery state monitoring circuit cannot maintain the over-discharge protective state at a given voltage, resulting in continuation of the discharge operation. The inability to maintain the over-discharge protective state adversely affects the battery lifetime, and thus the lifetime is short in the battery device using the conventional battery state monitoring circuit.
In addition, in the conventional battery device, because the voltage of xe2x88x92V0205 temporarily drops due to the coil component of an external load and so on, when the discharge current stops as soon as the over-current protective state is entered, the over-current protective state is caused to be immediately released. The operation principle is the same as that in the drawback of the over-discharge state. Accordingly, even if a large current flows in the switch circuit 203, the battery state monitoring circuit cannot maintain the over-current protective state at a given current, resulting in continuation of the discharge operation. The inability to maintain the over-discharge protective state also adversely affects the lifetime of the switch circuit, causing the lifetime to be short in the battery device using the conventional battery state monitoring circuit.
The present invention solves the above problems with the conventional battery device, and therefore an object of the present invention is to realize a battery state monitoring circuit which is capable of surely maintaining an over-charge protective state, an over-discharge protective state and an over-current protective state and to provide a battery device which is safe and has a long lifetime.
In order to achieve the above object, according to the present invention, there is provided a battery state monitoring circuit having a circuit that is capable of ensuring an over-charge release delay time, an over-discharge release delay time and an over-current release delay time.